>At 10:33 11/22/97 -0500, you wrote:
>>Besides not having 10 meters, is there any other diferences between the
>>SB221 and the SB220. Is the SB221 a newer amplifier, and if so, does it
>>include improvements that eliminated the parasitic problem.
>===============================================>
>Len.
>I'm sure you will get replies from others on this question, but here is
>what I think I know about the SB-220 vs. SB-221.
>
>Except for 10 meters being an after-add to the bandswitch wiring, they are
>the same amplifier, except....
- There is a CB filter in the SB-221, and some of the tuned input
circuits are different than the SB-220, due to the reactance of the CB
filter.
>Late versions of the SB-220 and, I suppose all of the SB-221 runs, had a
>different capacitof for C1 which had just a little bit wider plate spacing
>in the design. This was an attempt to stop arcing and spitting when the
>parasitics might occur.
>
The trade-off of wider spacing is that when the intermittent 110MHz
parasitic occurs, arcing tends to take place on the frangible bandswitch
contacts instead of across the tune capacitor. Bandswitch contacts tend
to evaporate during VHF-arcing. Tuning capacitor place don't.
>As for my SB-220 which I had for many years,
>
>AA)
>I put the old W4UX electronic bias circuit in it (QST; May, 1974).
>
>BB)
>Dropped the switch relay voltage (keying) down to just a few volts at just
>a few milliamps (Hints & Kink; QST, January, 1988),
>
>CC)
>Added a standby switch by putting a very small SPST switch in one of the
>small front panel holes that had one of those 4 small chrome headed screws
>in the four corners.
>(I've seen some pretty terrible standby switch installations in my hamfest
>tours)
>
The on/off switch works well as a standby switch in the SB-220. You
can't hurt the instant-on 3-500Z tubes because the inrush current is only
60% of what Eimac allows.
>DD)
>Put back-to-back 1N1004 diodes across the panel meters.
Better than zilch, but I would use 1N5400 series diode because they can
handle more peak current.
>BTW, when you load it up, do so in CW at 99 WPM.
>That way you limit the voltage driop from the power supply,
>and tune the amp for where it will normally be running, as opposed to
>loading it for a lesser voltage set of operating parameters.
>
Good advice. However, 99wpm is OK if your transceiver has a fast rise
time and a fast fall time. Otherwise, 60 or so wpm might work better.
>I never did have to replace the tubes, and I never had spitting during
>operation. The elimination of tube idling current between dits and dahs or
from
>riding along with voice modulation was a very wonderful add to the amplifier.
>
However, RF actuated bias switching is not so very wonderful to listen
to, both on frequency and on nearby frequencies. Switching a linear
amplifier back-and-forth between linear and non-linear bias during
transmit is not a wonderful idea. A better solution is to add high-speed
switching and run VOX. The result is much like talking on the telephone,
provided that one is using a modern high-speed switching radio.
>None of those other wonder adds were ever implemented into my amplifier.
>
If one never has VHF parasitic arcing, one probably doesn't need to
further reduce the VHF gain of the tubes. However, if the tubes are
ever replaced, the amplifier may get squirrely. Rumour has it that the
gain of the newer tubes is higher than in days of yore.
Rich---
R. L. Measures, 805-386-3734, AG6K
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